Combustion control is an important factor in optimizing fuel economy and performance in internal combustion engines. The amount of fuel introduced to the combustion chamber and the timing when that fuel is introduced contributes to the quality of combustion at given engine operating conditions. Fuel injectors are capable of introducing specific amounts of fuel at a given time by way of an actuation signal that originates in an engine controller. However, fuel injectors are multi-part mechanical components with moving pieces that exhibit performance variations from part to part, due to design factors and dimensional variances, even when the fuel injectors are made within specified manufacturing tolerances. When fuel injectors are activated with a nominal actuation signal the amount of fuel injected and the timing of when that fuel is introduced can be different from one injector to another injector.
There are known techniques for correcting performance variations in fuel injectors. These techniques address fuel injectors that introduce a liquid fuel, such as Diesel, to the combustion chamber. In a calibration phase during manufacturing, each fuel injector is actuated with a variety of actuation signals as a function of liquid rail pressure such that the actual quantity of fuel injected and other fuel injector characteristics can be measured and compared against set point values such that correction factors are identified. A bar code or other means on the fuel injector stores the correction factors, also known as trim information, such that when the fuel injector is installed in an engine the engine controller can be programmed with these values.
In the case of hydraulically actuated fuel injectors that introduce both a pilot fuel and a gaseous fuel, separately and independently, the quantity of gaseous fuel introduced by the injector and its timing is a function of more than just liquid rail pressure. For example, both the pilot (liquid) fuel rail pressure and gaseous fuel rail pressure influence injector performance. In liquid fuel injection systems the rail pressure is significantly higher than cylinder pressure in order to atomize the fuel during injection, for example diesel common rail pressure can be in the range 1000 bar to 1800 bar, and even higher. The differential pressure between in-cylinder pressure and liquid rail pressure is of a sufficiently large magnitude that the influence of in-cylinder pressure variations on injector performance is insignificant. However, when injecting a gaseous fuel directly into a combustion chamber, in-cylinder pressure variations can influence injector performance when gaseous fuel rail pressure is substantially less than liquid fuel rail pressure. There are a variety of reasons for designing a gaseous fuel injector to operate with a lower gaseous fuel rail pressure, for example between 100 bar and 500 bar. For instance, atomization is not required for a gaseous fuel so there is no motivation to increase gaseous rail pressure for this reason. Compressing a gaseous fuel, a compressible substance, requires more energy than compressing a liquid fuel, an incompressible substance, so the desire to maximize engine efficiency favors using a lower gaseous fuel rail pressure, as long as the pressure is high enough to inject the demanded quantity of fuel at corresponding engine operating conditions. It is with this objective in mind, that fuel injectors can be designed with the needed flow capacity at lower pressures. As a result, for many gaseous fuel injectors, the differential between in-cylinder pressure and gaseous rail pressure is of a smaller magnitude compared to typical liquid fuel injectors. As a result, for gaseous fuel injectors, in-cylinder pressure variations can be a factor in injector performance.
Other parameters influencing injector performance are hydraulic fluid pressure and hydraulic pulse width. When activated by the nominal actuation signal, hydraulic fluid pressure decreases inside the injector actuating mechanisms to inject fuel. Because of the aforementioned dimensional variances that are inevitably introduced during manufacturing, the injectors exhibit performance variations caused by changes in hydraulic fluid pressure, such as closing and opening times. As the desired on-time (hydraulic pulse width) for the injector decreases the variations in opening and closing time of the injector have an increased influence on the amount of fuel that is actually introduced. This influence is especially noticeable when the injector partially opens. The fuel flow area in an opened injector changes from injector to injector because of dimensional differences introduced by the manufacturing process which allows variations within specified tolerances. Therefore for identical hydraulic pulse widths (desired injector on-time) the amount of fuel that is actually introduced can be different from one injector to another injector even though both are manufactured in accordance with specifications.
Unlike a simpler monofuel injector that injects only one fuel, there are at least four parameters that influence fuel injector performance in a hydraulically-actuated gaseous fuel injector introducing both a pilot fuel and a gaseous fuel, separately and independently. These parameters are pilot fuel (liquid) rail pressure, gaseous fuel rail pressure, in-cylinder pressure and hydraulic pulse width. During the calibration phase using traditional liquid fuel trimming techniques, an increased number of test points are used for the gaseous fuel injector described above, due to the number of parameters influencing injector performance, compared to a conventional liquid fuel injector, resulting in a larger amount of fuel injector trim information.
Several techniques are known to store fuel injector trim information on the fuel injector that can later be programmed into an engine controller, such as on a bar code, a memory device or an integrated circuit. The information that needs to be stored can be accommodated by these techniques. Normally, during production the trim code is transferred to the engine controller by an automated method, such as by a bar code scanner or by RFID. There are times, however, when the trim code is entered manually, for example when a fuel injector is replaced in the field. Using conventional trimming techniques with the gaseous fuel injector described above resulted in large trim codes, due to the many test points used as a consequence of the multiple engine parameters influencing injector performance. Larger trim codes can be impractical and prone to error when entered manually by an operator.
U.S. Pat. No. 6,112,720, issued Sep. 5, 2000 to George M. Matta (the '720 patent) discloses a method of tuning hydraulically actuated fuel injectors based on electronic trim. The technique involves representing a difference in fuel delivery between a nominal fuel injector and an actual fuel injector as a linear relationship that is a function of liquid rail pressure. The nominal fuel injector is a theoretical perfectly performing injector without variations due to tolerancing or other manufacturing considerations. Since the relationship is assumed linear, two test conditions are used to determine equation (1) of the linear relationship from which constants a1 (y-intercept) and a2 (slope) are learned. The change in on time required for the actual fuel injector is then calculated according to equation (2) where the difference in fuel delivery is divided by the slope of the fuel delivery curve for the actual fuel injector. Since the slope of the actual fuel injector is not known the slope for the nominal fuel injector is employed instead. By substituting equation (1) into equation (2) the trimming solution, that is the adjustment in on-time for the actual fuel injector is derived according to equation (3).
The technique of the '720 patent has a number of approximations that introduce error into the trimming solution of equation (3) and limitations resulting in reduced injector performance. In a first approximation, in calculating the change in on-time for the actual injector according to equation (2) to compensate for the difference in fuel delivery between the actual and ideal injectors, the slope of the fuel delivery curve for the ideal (nominal) injector is employed instead of the slope of the fuel delivery curve for the actual injector which is not known. This introduces an error in the calculation since the correct slope to employ is that for the actual injector fuel delivery curve, which is different than the slope of the ideal injector fuel delivery curve. In a second approximation, a linear relationship is assumed to exist between the liquid rail pressure and the difference in fuel delivery between the ideal and actual injectors. As previously discussed, the performance of a hydraulically actuated fuel injector that injects a gaseous fuel, or a gaseous fuel and a liquid fuel, is dependent upon multiple engine operating parameters. Accordingly, the difference in fuel delivery of such a gaseous fuel injector and a nominal injector is not a simple linear relationship of engine operating conditions.
The '720 patent does not disclose any solution for correcting for differences in start of injection timing between the nominal fuel injector and actual fuel injectors. Errors in start of injection directly contribute to reduced combustion performance. The '720 patent does propose a technique for adjusting on-time of an actual fuel injector to correct for fuel delivery variations from the nominal injector. The technique does not correct for the non-linear behavior of fuel injector performance as the commanded on-time decreases and approaches the opening and closing times of the injector. Yet another limitation of the technique of '720 patent is the reliance upon an ideal (nominal) fuel injector as a starting position for fuel injector operation. An ideal (nominal) injector is employed to compare performance against an actual fuel injector and from which correction in on-time for the actual fuel injector is derived. In reality there is no such ideal fuel injector since the injectors exhibit dimensional variations due to tolerances allowed in manufacturing. In the event a trimming solution for an actual fuel injector is not found, for example trim information was not entered during a fuel injector replacement in the field then the on-time for the nominal injector is employed. However, the performance of an ideal injector is not the same as the performance of an average injector, for example the average injector from a lot of manufactured injectors.
The state of the art is lacking in techniques for generating fuel injector trim information during calibration of gaseous fuel injectors whose performance is influenced by a plurality of engine operating parameters. The present method and apparatus provide an improved technique for generating and using fuel injector trim information in an internal combustion engine.